According to the U.S. Center for Disease Control (CDC), heart disease is the leading cause of death in the United States, and atherosclerosis (a progressive narrowing and hardening of the arteries over time) is the leading factor in heart disease. Although initial thickening of arterial walls develops over a very long period of time, the complications from atherosclerosis generally occur suddenly and often prove fatal. Further, because atherosclerosis is asymptomatic, the first symptom perceived by a patient may be a life-threatening event.
Several methods are available for measuring the progress of atherosclerosis. These include Electron Beam Computed Tomography (EBCT), Magnetic Resonance Imaging (MRI), Flow Mediated Dilation (FMD), Quantitative Intima Media Thickness (QIMT), and the like. Although MRI may be use for clinical measurements, it remains a costly procedure. FMD appears promising, but outcome studies are not available to validate its predictive ability.
Carotid Intima Media Thickness (IMT) has been shown to be a useful surrogate endpoint. A surrogate endpoint is an indirect measurement which correlates with an actual (i.e., clinical) outcome. A clinical endpoint, for example, could be death, loss of vision, or other major symptomatic event. A corresponding surrogate endpoint could be a measurement which indicates the development of a condition which is correlated with a clinical endpoint event, but which surrogate endpoint may not produce easily observable symptoms. In particular, IMT measurements of the far wall of the carotid artery, about 1 cm below the bulb, have shown high correlation with cardiovascular clinical endpoints.
Carotid IMT provides a suitable surrogate endpoint for all stages of atherosclerosis and is well suited for tracking the progress of atherosclerosis. Carotid IMT has predictive value with respect to cerebrovascular and cardiovascular clinical events in both men and women. It has further been found that providing an ultrasound image of the carotid artery to a patient has a substantial effect of motivating changes to behavior, and the provision of subsequent images showing progress helps patients remain motivated to sustain a healthy life style.
Carotid IMT may be measured by either analog caliper IMT or digital Quantitative IMT (QIMT). Both methods rely on the same image and attempt to measure the same quantity. Caliper IMT is a manual measurement of the IMT visible in an ultrasound image using calipers, and is similar to printing a photo, and then measuring a dimension of a characteristic of the photograph by laying a ruler on the photograph. QIMT is a computer analysis of IMT performed on a digital version of the ultrasound image. Results have shown that caliper IMT has four times the standard deviation of QIMT when performed on the same images. Because of the accuracy of QIMT measurements, QIMT is ideal for both absolute measurements of the IMT, and for tracking small changes in the carotid artery, and thus the progression of atherosclerosis. The ability to track small changes allows the success or failure of a given course of treatment to be analyzed and adjusted if necessary, before a critical state is reached.
Current practice in the field of QIMT is to record carotid artery ultrasound images on SVHS tapes, and later manually review the images to determine a Region Of Interest (ROI) within each frame for subsequent computer analysis using edge detection algorithms. Such manual review requires highly skilled technicians and is time intensive. Because of the present requirement for manual review, IMT presently is only available from specialized laboratories employing highly trained technicians. There is thus a need for an objective system and method to automate the determination of the ROI of carotid ultrasound images, which method does not introduce operator bias.